The present invention relates generally to waveform synthesizers, and more particularly, to a digital waveform synthesizer that employs a direct digital synthesizer and digital frequency dividers.
An existing variable frequency oscillator (VFO) uses a combination of indirect frequency synthesis (phase lock loop frequency synthesizer) to provide frequency upconversion and multiplication, and a direct digital synthesizer using a triangular waveform to generate very fine frequency steps. The use of the phase lock loop synthesizer with very narrow bandwidth limits the switching speed performance of the this design. In addition, the phase lock loop scheme uses programmable feedback dividers that degrade the noise (and spurious signal) performance inside the phase lock loop bandwidth by a factor of 20*log(N), where N is the divider ratio, over the noise floor of both the frequency divider and the direct digital synthesizer. The phase lock loop also adds a noise hump near its loop bandwidth. Finally, the triangular waveform used in the direct digital synthesizer generates a higher spur level compared to commercially available direct digital synthesizers that generate a sinusoidal waveform.
The linear frequency modulated (FM) waveform for use in frequency modulated ranging (FMR) is generated by applying a linear voltage ramp derived from a linear voltage ramp generator to a voltage controlled crystal oscillator (VCXO). The output of the VCXO is frequency multiplied to X-band to provide maximum usable linear FM bandwidth of 100 KHz. The linearity and the slope accuracy is limited by the linearity of the VCXO and the accuracy of the voltage ramp generator. The slope accuracy is increased but at a cost of using multi-unit level calibration and a microwave calibration target source. Also, the VCXO requires approximately 1 KHz of linear frequency tuning which may be achieved by degrading its FM noise performance.
Accordingly, it is an objective of the present invention to provide a direct digital synthesizer that overcomes the limitations of the above-described conventional waveform synthesizers. In particular, it is an objective of the present invention to provide a direct digital synthesizer that provides fast switching, flexible and precisely defined signals, and that improves the inherently poor spurious signal performance of its direct digital synthesizer.